1. Technical Field
The present disclosure relates to power converter circuits, in particular to a control circuit implementing a related method for controlling a switching power factor corrector (PFC), a PFC and an AC/DC converter including the PFC.
2. Description of the Related Art
Many electronic devices, such as computers, TV sets, etc. or rechargeable batteries, use a DC voltage as supply voltage, thus the AC voltage made available by the mains is AC/DC converted. Especially in cases in which the power consumption of the supplied load is relatively great, a power factor correction is used to minimize reactive power and to maximize real power absorbed from the mains. In this case, an AC/DC converter that provides power factor correction includes a power factor corrector (PFC) and eventually a DC/DC converter in cascade to the PFC.
A PFC of the type shown in FIG. 1 usually comprises a bridge rectifier that rectifies the mains voltage and generates a rectified input voltage Vin, a core PFC_PLANT that generates a DC output voltage Vout, and a control circuit CONTROL LOOP that senses the output voltage Vout, the rectified voltage Vin and the current flowing throughout the inductance and generates a control signal for turning on/off the power switch SW of the converter PFC_PLANT. The core PFC_PLANT may be controlled in continuous current mode (CCM) and the current absorbed from the mains is substantially sinusoidal and in phase with the mains voltage, as schematically shown by the exemplary time graphs of FIG. 2 of the current through the inductor (1), of the low-frequency component of the current through the inductor (2), of the current through the switch (3), of the current through the diode (4) and of the control signal of the switch SW (5), obtained through simulation of the PFC of FIG. 1. The most recent norms and International standards specify high efficiency and good power factor (PF) across the entire load range. An objective to be attained in this field is to define a novel PFC control methodology able to improve efficiency at light load conditions together with a low total harmonic distortion (THD) across the entire load range.
It is possible to control the PFC in CCM with an average current mode control with constant switching frequency. Such a control technique is well known in the art. The average current mode can be implemented by sensing the input voltage and generating a reference signal by means of multiplier circuit or without input line sensing. This second approach is used in the devices UCC28180 of Texas Instruments and ICE1PCS01 of Infineon, to the datasheets of which the skilled person is addressed for details. In both cases, a feedback control loop of a CCM PFC processes a sense signal Isense representative of the current flowing throughout the inductor L. One main feature of the average current-mode control is the presence of two control loops, the voltage loop and the current loop. Both of them require a proper compensation network.
The constant frequency average current mode control has also a very low efficiency at light load due to high switching losses. Besides, average current mode controllers present a non-linear characteristic at light load that increases harmonic distortion in discontinuous current mode (DCM) operation.
Another control technique is implemented in the devices PFS Hiper of the Power Integrations. In these devices the switching frequency is variable and the duration of the on-time and off-time are defined by means of a constant amp-seconds on-time control, and constant volt-seconds off-time control respectively. The skilled person is addressed to the datasheet of the above device for further details. A drawback of these devices is the very wide switching frequency variation with rms input voltage range and input voltage conduction angle. In particular, the switching frequency with high input voltage (230AC) when the line conduction angle is near to 90 degree is very low. Such a frequency profile is risky since it could introduce EMI input filter oscillations.
The article by Qian Li, Fred C. Lee, Ming Xu and Chuanyun Wang, “Light load efficiency improvement for PFC”, Energy Conversion Congress and Exposition, 2009. ECCE 2009. IEEE, 20-24 Sep. 2009, pages 3755-3760, discloses a digital system for controlling CCM PFCs at light load conditions with an adaptive constant on-time control technique. Adaptive Constant on time has many attractive features as the automatic reduction of switching frequency, resulting improved light load efficiency. Anyway, the method described in this prior article suggests to calculate the off-time by means of a digital predictive method. Unfortunately, this algorithm is relatively complicated and a digital system (DSP microprocessor and fast A/D converter) is needed to sample the input voltage and the current through the inductor and to calculate the most appropriate duration of the next off-time. This solution is not very appealing because it increases overall costs of the PFC.